Radiation backscatter measuring instrument

ABSTRACT

A radiation backscatter measuring instrument is provided for use with a portable probe assembly having an elongated casing with an opening at one end thereof and containing a radiation source positioned in predetermined relation therewith and a radiation detector for detecting radiation backscattered from a workpiece exposed to radiation from the source through the casing opening. The radiation backscatter measuring instrument includes a planar, horizontally disposed, transparent locator with a series of guide lines on the bottom surface thereof which is positioned over the selected locus of measurement on the horizontally supported workpiece. The locator is then horizontally retracted from its locating position while the portable probe is vertically lowered to engage the workpiece with the opening thereof being positioned over the selected portion of the workpiece to be tested.

United States Patent ]Match 13, 1973 9/1970 Joffe et al. ..250/l06 S XPrimary ExaminerJames W. Lawrence Assistant Examiner-D. L. WillisAttorney -Robert E. lsner and Peter J. Franco [57] ABSTRACT A radiationback scatter measuring instrument is provided for use with a portableprobe assembly having an elongated casing with an opening at one endthereof and containing a radiation source positioned in predeterminedrelation therewith and a radiation detector for detecting radiationbackscattered from a workpiece exposed to radiation from the sourcethrough the casing opening. The radiation backscatter measuringinstrument includes a planar, horizontally disposed, transparent locatorwith a series of guide lines on the bottom surface thereof which ispositioned over the selected locus of measurement on the horizontallysupported workpiece. The locator is then horizontaily retracted from itslocating position while the portable probe is vertically lowered toengage the workpiece with the opening thereof being positioned over theselected portion of the workpiece to be tested.

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SHEET 2 OF 3 I q. INVENTOR.

WILLIAM D. HAY BY 1 Z ATTORNEY PATENTEMRl 3191s saw a or 5 II lI/lINVENTOR.

, HAY.

WILLIAM 0 ATTORNEY RADIATION BACKSCATTER MEASURING INSTRUMENT Thisinvention relates generally to instruments employing beta-radiationbackscatter techniques for measuring the thicknesses of thin layers orcoatings of one substance on a base material or substrate having asignificantly different atomic number.

In instruments of the described type, radiation from a suitablebeta-emitting isotope is directed against a surface of a workpiece orsample to be measured, and a detector, such as a Geiger tube, picks upthe backscatter or beta-radiation back-scattered from the workpiece, andtransmits corresponding signals to a suitable electronic system forinterpretation and translation into readings on a meter. The intensityof the backscatter of the beta rays is proportional to the atomic numberof the material at the surface of the workpiece against which theradiation is directed and, where the workpiece has a coating substanceof an atomic number different than that of the base material orsubstrate, the intensity of the backscattered radiation is furtherproportional to the thickness of the coating.

Comparisons of the values obtained by the above procedure with thevalues from known thickness standards lead to quantitative measuredresults. Thus, through such comparative measurement techniques, themeter readings can be calibrated to indicate the thicknesses of variouscoating substances on various base materials. Beta-ray backscattermeasuring instruments have particularly found application in themeasurement of the thicknesses of thin precious metal depdsits and othercoatings on small or restricted areas of electronic components such asprinted circuit boards and the like.

Only the coated area can be exposed to radiation if the thicknessmeasurement thereof is to be accurate and reproducible since thebackscatter of radiation from adjacent uncoated surfaces of the basematerial will result in an erroneous thickness measurement. Accurate andreproducible measurements of the thicknesses of such layers or coatingshave been achieved for coatings which extend linearly for a substantialdistance with the apparatus disclosed in U.S. Pat. No. 3,496,359 whichissued on Feb. 17, 1970. Where, however, the coatings have an irregularconfiguration such as at contact fingers, small pads and other suchareas which do not have a substantial linear component it has provendifficult to obtain an accurate and reproducible thickness measurementsince the areas of the particular coatings to be measured cannot beprecisely located.

This invention may be briefly described as an instrument employing betaradiation backscatter techniques for measuring the thicknesses of thinlayers or coatings of one substance on a base material or substratehaving a significantly different atomic number wherein a transparentplastic hair line alignor is horizontally positioned over and definesthe locus of measurement or the precise area of the coating to be testedand is then horizontally retracted with a pencil probe unit beingvertically lowered into engagement with the workpiece so that theopening through which beta rays pass from the probe unit is positionedover the area or locus located by the alignor.

Among the advantages of the present invention is the provision of aninstrument which can measure the thicknesses of thin layers or coatingsof one substance on a base material or substrate having a significantlydifferent atomic number whether the layer or coating is regular orirregular, linear or curvilinear and whether the layer is tested at theterminus thereof or measured at a position along its length.

Accordingly the principle object of the present invention is theprovision of an improved radiation backscatter measuring instrument thatincludes a guide or locator which can be positioned to define theprecise area of the coating whose thickness is to be measured.

Another object of the present invention is the provision of a radiationbackscatter measuring instrument in which the probe assembly is biasedinto engagement with the workpiece being tested and the magnitude of thebiasing force is limited so that neither the workpiece nor the measuringinstrument can be damaged during the testing operation.

Other objects and advantages of the present invention will becomeapparent from the following portion of the specification and from theaccompanying drawings which illustrate, in accord with the mandate ofthe patent statutes, a presently preferred embodiment incorporating theprinciples of the invention.

Referring to the drawings:

FIG. 1 is a side elevational cross sectional view of the radiationbackscatter measuring instrument of the present invention.

FIG. 2 is a view of the measuring instrument as constituted at the line2-2 of FIG. 1.

FIG. 3 is a cross sectional view of the measuring instrument asconstituted at the line 3-3 of FIG. 1.

FIG. 4 is a view of the measuring instrument as constituted at the line4-4 of FIG. 9.

FIG. 5 is a view of the measuring instrument as constituted at the line55 of FIG. 1.

FIG. 6 is a view similar to that of FIG. 5 with the portable probeassembly lowered into engagement with a circuit board workpiece.

FIG. 7 is a view of the measuring instrument as constituted at the line77 of FIG. 1.

' FIG. 8 is a view of the measuring instrument similar to FIG. 1 withthe portable probe assembly lowered into engagement with the circuitboard workpiece.

FIG. 9 is a view of the measuring instrument similar to FIG. 8 with theportable probe assembly biased into forceful engagement with the circuitboard workpiece.

FIG. 10 is a perspective view of the measuring apparatus elevated out ofengagement with a standardizing instrument.

FIG. 11 is a top view of the measuring instrument inverted and mountedon a stand to be utilized when the portable probe assembly opening andlocator guide lines are aligned.

FIG. 12 is an elevational view of an alignment checker utilized inconjunction with the measuring instrument as illustrated in FIG. 11.

FIG. 13 is a view of the measuring instrument as constituted at the line1313 of FIG. 1.

Referring to the drawings, and initially to FIGS. 1 through 4, aninstrument 20 employing beta radiation backscatter techniques formeasuring the thicknesses of thin layers or coatings of one substance ona base material or substrate having a significantly different atomicnumber is illustrated. The measuring apparatus includes a housing 22which has a central guide body 24 fixedly secured to a pair of opposingside walls 26 (FIG. A horizontal bore 28 extends completely across theguide body and a pinion member 30 is rotationally supported by theopposing side walls and positioned therein. A pair of knobs 32 controlthe movement of the pinion member.

Two vertically extending bores are defined in the guide body. A first ofthese vertical bores 34 extends from the top to the bottom of the guidebody and is selectively sized and shaped so as to slidingly receive aportable probe assembly 36. The second of these vertical bores 38 whichalso extends the entire vertical dimension of the guide body isselectively sized and shaped so as to slidingly receive a toothed rackmember 40 which engages the pinion 30 and is controlled thereby.

The portable probe assembly 36 is mounted on the toothed rack of therack and pinion assembly by means of a coupling 44. The coupling 44 hasfirst 46 and second 48 cylindrical bores which extend from the topsurface to the bottom surface thereof. A vertically extending slit 50 isestablished in the coupling and extends vertically from the top to thebottom surface thereof and horizontally from one side thereof, throughone of the bores 48 to the other bore 46 thereby defining opposingcoupling portions which are integral at the other end thereof. Thecoupling is made from a resilient material and the two coupling portionsmay be drawn together, with the solid end of the coupling functioning asa hinge, by a bolt 47 which extends through one portion of the couplingand is threadedly engaged with the other coupling portion. The firstbore 46 is selectively sized so that when the two coupling portions aredrawn together subsequent to the positioning of the probe assemblytherewithin, the probe assembly will be compressively located withinthis first coupling bore 46 and will become integral therewith. Thesecond coupling bore 48 is selectively sized so as to allow verticaldisplacement of the toothed rack therethrough when the coupling isintegral with the probe assembly. The two coupling portions may beseparate pieces which are hingedly joined.

The toothed rack has a horizontally extending V- shaped notch 52 andeach coupling portion has a horizontal slot 54 which extends in adirection substantially parallel to the base of the V-shaped notch and apair of bores 56 which extend perpendicularly thereto establishing apair of continuous bores extending through the coupling from one slot tothe other. A bar 58 is positioned in one slot and is attached by a pairof springs 60 which extend through the aligned bores 56 of the couplingportions to a rolling pin 62 which is positioned in the other slot. Theslot in which the rolling pin is positioned has sufficient depth so thatthe rolling pin which is forcefully urged by the springs against therack member can roll up the inclined surface of the V-shaped notch andalong the outer surface of the rack member when the coupling isdisplaced vertically relative to the toothed rack. Such relativedisplacement may occur when the displacement of the portable probeassembly is blocked. When the portable probe assembly is free to movethe compressive force exerted by the springs is sufficient to maintainthe rolling pin proximate the base of the V-shaped notch 52 andaccordingly the portable probe assembly and the rack will then bedisplaced as a unit. The top of the rack has an inclined portion 63 toenable the integral portable probe and coupling members to be easilymounted on the toothed rack.

An axle supporting member 64 is secured to the bottom of the toothedrack and a wheel 66 is rotationally mounted to this axle 68. The axlesupporting member has a stop 70 which projects horizontally therefromtowards the rear of the guide body.

The instrument includes a wedge shaped member 72 which has a cammingsurface 74 extending downwardly toward the front of the guide body. Apair of wings 76 extend outwardly from either side and rearwardly fromthe back of the wedge shaped member and are slidingly received in a pairof opposing grooves 78 defined in the guide body (FIGS. 11 and 13).These grooves extend horizontally towards either side of the guide bodyfrom a channel 80 which extends substantially from the front to the rearof the guide body. The'grooves precisely mate with the wings 76 of thewedge shaped member 72 and accordingly restrict the motion thereof to asingle direction from the front to the back of the guide body.

A base plate 82 is secured to the bottom of the wedge shaped member 72by a plurality of screws 84 and a planar, locator is secured in ahorizontal disposition to the bottom surface of the base plate by aplurality of screws 94. A spring element 96 which extends between therearward most point of the wings of the wedge shaped element and a pin98 which is integral with the guide body urges the wedge shaped elementforward toward the front of the guide body and accordingly results inthe wheel 66 rolling up the camming surface thereby elevating thetoothed rack and probe assembly until an upwardly extending post 95, inthe form of a bolt, which is screwed into a threaded bore defined in theguide body and which projects downwardly through a slotted guide 97 inthe wings of the wedge shaped element is stopped by abutting against oneend thereof. The forwardmost or home position of the locator 90 isthereby determined.

The locator 90 (FIG. 5) is transparent and has a series ofperpendicularly extending guide lines 92 on the bottom surface thereofand the height of the side walls 26 of the instrument is selectivelychosen so that this surface will lie substantially within the planedefined by the bottom surfaces thereof. The entire instrument is placedon top of a circuit board workpiece 93 with the guide lines 92 of thelocator locating a locus of measurement. The locator can be preciselypositioned over any locus of measurement whether it is a portion of apad, line (linear or curvelinear) or contact finger.

I The portable probe assembly 36 is a conventional unit having anelongated casing with a bottom facing plate or mask 112 having arectangular opening 114 therein (FIGS. 1, 6 and 7). The casing includesa radiation source which is positioned in predetermined relation withthe opening 114 and a radiation detector such as a Geiger-Muller tubefor detecting radiation which is emitted from the source through theopening and backscattered therethrough from the workpiece area which isexposed to the radiation. The detected radiation is translated intoutilizable intelligence by a readout meter 115 (FIG. While the openingas illustrated is rectangular it could be square, circular or have anyother common geometric configuration to suit the particular needs of anygiven situation.

When the locator is in the home position the center of the locus ofmeasurement defined by the guide lines (FIG. 5) lies directly below thecenter of the opening of the vertically supported portable probeassembly. Accordingly as the locator is horizontally retracted bylowering the toothed rack, the portable probe will be lowered intoengagement with the circuit board with the center of the opening thereofabove the center of the specific locus of measurement located by thelocator element (FIG. 6).

The bottom face plate or mask 112 (FIG. 7) of the portable probeissecured to the probe casing by three screws 116 which extend throughapertures 118 defined therein. Sufficient tolerance is provided so thatthe mask can be adjusted a limited extent to vary the alignment of theopening.

As can be seen when reference is had to FIGS. 8 and 9 the portable probe36 is lowered into engagement with the circuit board workpiece 93 priorto the lowering of the toothed rack to its lowermost position. When thedownward displacement of the portable probe assembly is blocked by thecircuit board workpiece the further rotation of the control knobs 32results in the further lowering of the toothed rack 40 with the rollerbar 62 rolling up the surface of the V-shaped notch 52 until the stop 70of the axle supporting member 64 engages the top surface of the wings 76of the wedge shaped member as illustrated in FIG. 9. The downwarddisplacement of the rack is thereby limited so that it will not impactagainst the transparent locator. As can be seen the toothed rack istherefore displaced the selected distance a (FIG. 4) relative to thestationary portable probe assembly and when manually maintained at itslowermost position the stretched springs 60 exert a force which urgesthe portable probe assembly downwardly into compressive engagement withthe circuit board workpiece. The force with which the portable probeassembly is urged into such engagement is independent of the operatorand is solely dependent upon the strength of the spring element and theextent of stretching thereof when the toothed rack is displaced relativeto the probe assembly the preselected distance a. Accordingly no harm toeither the circuit board workpiece coatings or the portable probeassembly can result as a result of the operation of the instrument.

In order to properly standardize the testing apparatus a standardizingtool is provided which has a flat platform 120 with two locating pins122 projecting upwardly therefrom and a depression 124 at one endthereof which has a sample supporting pin (not shown) projectingupwardly therefrom (FIG. 10). A sample of known thickness 126 is mountedon a cylindrical base 128 and is positioned on the samplesupporting pin.The composite thickness of the sample 126 and the circular base 128 is aconstant and is selectively chosen so that when the composite element ispositioned on the pin with the top surface of the sample perfectlyhorizontal the top surface will lie in substantially the same place asthe top surface of the platform. The measuring instrument is placed onthe platform 120 with the locator positioned over the sample. Since thebottom surface of the locator lies in the plane defined by the bottomsurfaces of the side walls 26 or the top surface of the platform thecomposite member will be adjusted by engagement with the locator untilthe top surface thereof also lies substantially in the same plane.Standardization of the measuring instrument can then be effected.

To assure that the readings obtained will not vary due to the varyingalignment of the opening of the portable probe assembly relative to theguide lines of the locator a method of aligning same may be practicedwith the aligning apparatus of the present invention. This apparatusincludes an alignment checker 132 (FIGS. 11 and 12).

The alignment checker (FIG. 12) has a pair of planar legs secured byscrews 142 to an elongated rectangular block 144. A bore 146 extendstherethrough in a direction parallel to the legs and perpendicular tothe top surface of the block. The bore is selectively sized so as tofrictionally engage the portable probe assembly when it is insertedthereinto while allowing movement of the probe therethrough whensufficient force is applied to overcome these frictional forces. The topsurface of the block includes a planar cutout 152 and a transparentplate 154 is placed therein and secured to the block by means of screws156. The bottom surface of the transparent plate has a pair ofperpendicularly extending cross-hairs marked thereon which extendprecisely transversely and laterally across the plate. The portion ofthe alignment checker directly beneath the transparent plate is cut awaythereby defining an access chamber 158.

To properly establish the alignment of the locator guide lines 92 andthe mask opening 1 14 the measuring apparatus is inverted. With thelocator member in its fully advanced position (FIG. 1) the alignmentchecker is positioned so that the locator member extends into the accesschamber 158 and the alignment checker bore is coaxial with the elongatedcasing of the portable probe assembly. The locator member is inaintainedat its fully advanced position and the portable probe assembly isdisplaced relative to the stationary rack member towards its advancedposition until the probe mask engages the locator member.

The fictional engagement of the portable probe casing and the alignmentchecker bore maintains the horizontal alignment of the transparent plate154 of the alignment checker and when so engaged the planar legs 140thereof contact the front face of the guide body 24 preventing therotation of the alignment checker about the probe casing therebymaintaining the lateral and transverse alignment of the alignmentchecker cross hairs.

As can be seen in FIG. 1 a limited degree of then further advanced untilthe mask thereof engages the bottom face of the transparent plate 154 ofthe alignment checker.

The transparent plate includes three apertures 160 (FIG. 11) which areselectively located to lie above the bottom facing plate screws 116 ofthe probe assembly. The bottom facing plate screws can thereby beloosened and the plate can be manipulated within the block accesschamber 158 until the opening 114 thereof is precisely aligned with thecross-hairs of the alignment checker and accordingly with the guidelines of the locator. Tightening these screws will then maintain thedesired alignment thereof.

As an aid to the accurate positioning of the locator member over thedesired locus of measurement, a pair of lights 162 are provided (FIG.13) for increasing the visibility of the circuit board workpiece and amagnifying lens 164 is suitably secured to the guide body (FIG. 1) toprovide an enlarged view of the coating portion being measured.

Having thus described my invention, what I claim is:

l. A radiation backscatter measuring instrument for use with a probeassembly having a vertically disposed elongated casing with an openingat the bottom thereof and containing a radiation source positioned inpredetermined relation therewith and a radiation detector for detectingradiation backscattered from a horizontally supported workpiece exposedto radiation from the source through the casing opening comprising meansfor maintaining the vertically disposed elongated casing of the probeassembly in a retracted position spaced from the workpiece,

locator means including a transparent, horizontally disposed memberhaving guide lines thereon for precisely locating a locus of measurementon the workpiece, means for maintaining said locator means with saidtransparent member in an advanced position proximate the workpiece andvertically below the opening in the elongated probe assembly casing,

means for horizontally displacing said locator means member from theadvanced position thereof to a retracted position, and

means for displacing the probe assembly casing downwardly from theretracted position thereof to an advanced position wherein the probeassembly casing engages the workpiece and the casing opening is situatedabove the locus of measurement located by said locator means.

2. A radiation backscatter measuring instrument according to claim 1,wherein said probe assembly casing displacing means further comprisesmeans for biasing the probe assembly casing against the workpiece andmeans for limiting the magnitude of the force exerted by said biasingmeans when the probe assembly casing is in the advanced position.

3. A radiation backscatter measuring instrument according to claim 2,wherein said probe assembly casing displacing means includes rack andpinion means and means for integrally coupling said rack means to theelongated casing of the probe assembly during the displacement of theprobe assembly casing from the retracted to the advanced positionthereof.

4. A radiation backscatter measuring instrument according to claim 3,wherein said coupling means further comprises means for allowing thedisplacement of said rack means from a first position wherein the probeassembly casing is in the advanced position engaging the workpiece,downwardly a substantial distance to a second position, said allowingmeans including said means for biasing the probe assembly against theworkpiece being tested.

5. A radiation backscatter measuring instrument according to claim 4further comprising body means adapted to be positioned over theworkpiece including first vertically extending bore means selectivelysized for slidably receiving the vertically disposed elongated casing ofthe portable probe assembly and second vertically extending bore meansfor slidably receiving said rack means.

6. A radiation backscatter measuring instrument according to claim 5wherein said rack means comprises said means for displacing said locatormeans.

7. A radiation backscatter measuring instrument according to claim 6wherein said coupling means includes first and second portionsresiliently joined at one end thereof, means for drawing one of saidportions towards said other one of said portions, and 1 first and secondbore means vertically extending from the top surface to the bottomsurface thereof and each being defined in said first and secondportions, said bores being selectively sized so that when said twoportions are so drawn together, the portable probe assembly can becompressively secured within one of said bores and said rack means canbe slidably received within said other bore.

8. A radiation backscatter measuring instrument according to claim 7wherein said rack means has a V- shaped notch defined therein with thebase thereof extending horizontally thereacross, and said coupling meansfurther comprises first and second opposing slot means extendingsubstantially parallel to the base of said notch, one of said slot meanscommunicating with the other one of said bore means, first and secondpin means positioned within said slots, spring means connecting saidfirst and second pin means for forcefully pulling said first pin meanstowards said second pin means, said one of said slots being selectivelysized so as to allow one of said pin means to follow the inclinedsurface of said notch and the outer surface of said rack means as saidrack means is displaced relative to the probe assembly.

9. A radiation backscatter measuring instrument according to claim 8,wherein said locator means further includes a horizontally displaceablecam means having a camming surface defining an angle with the axis ofsaid rack means and said rack means further includes follower means forengaging said camming surface whereby the vertical lowering of said rackmeans towards the workpiece will horizontally displace said locatormember away from the locus of measurement of the workpiece.

10. A radiation backscatter measuring instrument according to claim 5further comprising alignment checker means including block means,

9 l transparent plate means including perpendicularly of said bore meansand selectively located so that extending crossrhairs thereon when theradiation backscatter measuring instruaccess chamber means intermediatesaid plate means ment is inverted and the alignment checker means andsaid block mean is displaced so that the probe casing is inserted intobore means extending substantially perpendicular to the bore of Saidalignment checker means, Said leg means will engage said body means forpreventing the rotation of said alignment checker means about the probecasing.

said plate means and communicating with said access chamber means andselectively sized to frictionally engage the probe casing, and

leg means extending in a direction parallel to the axis

1. A radiation backscatter measuring instrument for use with a probeassembly having a vertically disposed elongated casing with an openingat the bottom thereof and containing a radiation source positioned inpredetermined relation therewith and a radiation detector for detectingradiation backscattered from a horizontally supported workpiece exposedto radiation from the source through the casing opening comprising meansfor maintaining the vertically disposed elongated casing of the probeassembly in a retracted position spaced from the workpiece, locatormeans including a transparent, horizontally disposed member having guidelines thereon for precisely locating a locus of measurement on theworkpiece, means for maintaining said locator means with saidtransparent member in an advanced position proximate the workpiece andvertically below the opening in the elongated probe assembly casing,means for horizontally displacing said locator means member from theadvanced position thereof to a retracted position, and means fordisplacing the probe assembly casing downwardly from the retractedposition thereof to an advanced position wherein the probe assemblycasing engages the workpiece and the casing opening is situated abovethe locus of measurement located by said locator means.
 1. A radiationbackscatter measuring instrument for use with a probe assembly having avertically disposed elongated casing with an opening at the bottomthereof and containing a radiation source positioned in predeterminedrelation therewith and a radiation detector for detecting radiationbackscattered from a horizontally supported workpiece exposed toradiation from the source through the casing opening comprising meansfor maintaining the vertically disposed elongated casing of the probeassembly in a retracted position spaced from the workpiece, locatormeans including a transparent, horizontally disposed member having guidelines thereon for precisely locating a locus of measurement on theworkpiece, means for maintaining said locator means with saidtransparent member in an advanced position proximate the workpiece andvertically below the opening in the elongated probe assembly casing,means for horizontally displacing said locator means member from theadvanced position thereof to a retracted position, and means fordisplacing the probe assembly casing downwardly from the retractedposition thereof to an advanced position wherein the probe assemblycasing engages the workpiece and the casing opening is situated abovethe locus of measurement located by said locator means.
 2. A radiationbackscatter measuring instrument according to claim 1, wherein saidprobe assembly casing displacing means further comprises means forbiasing the probe assembly casing against the workpiece and means forlimiting the magnitude of the force exerted by said biasing means whenthe probe assembly casing is in the advAnced position.
 3. A radiationbackscatter measuring instrument according to claim 2, wherein saidprobe assembly casing displacing means includes rack and pinion meansand means for integrally coupling said rack means to the elongatedcasing of the probe assembly during the displacement of the probeassembly casing from the retracted to the advanced position thereof. 4.A radiation backscatter measuring instrument according to claim 3,wherein said coupling means further comprises means for allowing thedisplacement of said rack means from a first position wherein the probeassembly casing is in the advanced position engaging the workpiece,downwardly a substantial distance to a second position, said allowingmeans including said means for biasing the probe assembly against theworkpiece being tested.
 5. A radiation backscatter measuring instrumentaccording to claim 4 further comprising body means adapted to bepositioned over the workpiece including first vertically extending boremeans selectively sized for slidably receiving the vertically disposedelongated casing of the portable probe assembly and second verticallyextending bore means for slidably receiving said rack means.
 6. Aradiation backscatter measuring instrument according to claim 5 whereinsaid rack means comprises said means for displacing said locator means.7. A radiation backscatter measuring instrument according to claim 6wherein said coupling means includes first and second portionsresiliently joined at one end thereof, means for drawing one of saidportions towards said other one of said portions, and first and secondbore means vertically extending from the top surface to the bottomsurface thereof and each being defined in said first and secondportions, said bores being selectively sized so that when said twoportions are so drawn together, the portable probe assembly can becompressively secured within one of said bores and said rack means canbe slidably received within said other bore.
 8. A radiation backscattermeasuring instrument according to claim 7 wherein said rack means has aV-shaped notch defined therein with the base thereof extendinghorizontally thereacross, and said coupling means further comprisesfirst and second opposing slot means extending substantially parallel tothe base of said notch, one of said slot means communicating with theother one of said bore means, first and second pin means positionedwithin said slots, spring means connecting said first and second pinmeans for forcefully pulling said first pin means towards said secondpin means, said one of said slots being selectively sized so as to allowone of said pin means to follow the inclined surface of said notch andthe outer surface of said rack means as said rack means is displacedrelative to the probe assembly.
 9. A radiation backscatter measuringinstrument according to claim 8, wherein said locator means furtherincludes a horizontally displaceable cam means having a camming surfacedefining an angle with the axis of said rack means and said rack meansfurther includes follower means for engaging said camming surfacewhereby the vertical lowering of said rack means towards the workpiecewill horizontally displace said locator member away from the locus ofmeasurement of the workpiece.